OK well not really true telemetry but I wanted to tell you about a simple receiver battery monitor that my buddy designed for me. It uses an LED mounted to the plane that glows bright when the receiver battery voltage is up but dims when it is too low (there is plenty of redundant power left to get the plane back on the ground). Mounted under the wing it should tell me during a low slow flyby if my battery voltage is high enough. Cost was totally minimal got the parts from Radioshack.I wanted a simple way to check my receiver Lipo without hooking up my volt meter. I dont see much offered to achieve this commercially.

My buddy designed a very simple circuit that would shut off the light when the power dropped to a certain level, I am sure he could design something that would also turn on an LED when it was over a preset voltage. You would just have to wire in both. The weight is minimal and the current required is very small.